"My friend's I am here to warn you that should you ever stumble across a website with top notch "content" on the front page that just so happens to be sorted by a rating system, please. Please, for the love of all that is good in this world. DO NOT GO TO THE LAST PAGE! What has been seen, cannot be unseen."

The first and simplest thing to do is try to use the internet with a different browser. If you're using IE or Edge, try Chrome or Firefox, visa versa whatever. As Slick pointed out it could be an issue with hardware acceleration. You Tube dropped Flash and started using HTML5 back in early 2015, so this could be an issue with the video engine included with whatever browser you're running.

Also, as IR noted, it's never a bad idea to check for subsystem stability (i.e. RAM, CPU, hard drives) using the commonly available tools.